1. Field of the Invention
The invention concerns a device for roll and yaw attitude control of a satellite, usually a geostationary satellite, stabilized on its three axes, possibly including compensation of disturbing torque acting on the satellite about said three axes.
In this context a satellite is any artificial object in the solar system:
orbiting the Earth or any other planet in the solar system, or
orbiting a satellite of any planet in the solar system, or
in solar orbit, possibly a transfer orbit between two planets.
2. Description of the Prior Art
The attitude of an orbiting satellite is affected by disturbing torques, the major causes of which are:
the asymmetry of the solar radiation pressure due to the angle of the pitch axis (Y) of the satellite relative to the Sun (difference with respect to 90.degree.), to the differing reflectivity of different parts of the satellite and to any geometrical asymmetry of the satellite,
the local (for example, terrestrial) magnetic field,
the local (for example, terrestrial) gravity gradient (in low orbits),
the aerodynamics of the environment (in low orbits).
Consequently, a system for controlling the attitude of a satellite in its orbit is essential. Known active systems for this purpose respectively use multiple reaction wheels or thrusters, but:
the use of thrusters to impart brief attitude correction pulses from time to time causes excitations which affect the structure, the solar panels and the sensitive part of the satellite; also, the propellants used for attitude control increase the mass of the satellite;
the use of reaction wheels also increases the overall mass of the satellite.
To control the attitude of a satellite with minimum disturbance and with an optimized mass budget, use is made of disturbing forces due:
to the solar radiation pressure (by appropriately orienting surfaces attached to the satellite), or
to the local (for example, terrestrial) magnetic field, by creating magnetic dipoles on board the satellite using current loops.
Various documents have already proposed the use of solar pressure for satellite attitude control or station-keeping, using surfaces adapted to be oriented by dedicated actuators.
Representative prior art includes:
French patent FR-2.513.589: PROCEDE ET DISPOSITIF POUR ALIGNER L'AXE DE ROULIS D'UN SATELLITE AVEC UNE DIRECTION DESIREE,
German patent DE-2.537.577: LAGERREGELUNG FuR SATELLITEN,
French patent FR-2.550.757: REGULATION DE POSITION DE SATELLITES,
American patent US-3.304.028: ATTITUDE CONTROL FOR SPACECRAFT,
French patent FR-2.529.166: PROCEDE DE MAINTIEN EN POSITION D'UN SATELLITE
LA NAVIGATION A L'AIDE DE VOILE SOLAIRE ET VEHICULE SPATIAL METTANT EN OEUVRE LE PROCEDE.
These solutions increase the mass and reduce the reliability of the satellite.
A simpler way to control the attitude of a satellite stabilized on three axes is to orient appropriately the surfaces of the solar generator wings (which are generally elongated structures parallel to the pitch axis), using their respective drive motors, to create torques about two perpendicular axes in a plane (plane of the trajectory) perpendicular to the pitch axis Y: the projected solar axis S and the perpendicular transverse axis P, the three axes Y, S and P defining a direct trihedron.
This principle is described in particular in the documents:
ATTITUDE CONTROL BY SOLAR SAILING--A PROMISING EXPERIMENT ON OTS 2 by Udo RENNER--ESA JOURNAL 1979, Vol 3.
ONE YEAR OF SOLAR SAILING WITH OTS--ESA BULLETIN 31 - August 1982.
Patent FR-2.531.547: SYSTEME DE CONTROLE D'ATTITUDE D'UN SATELLITE GEOSTATIONNAIRE.
U.S. Pat. No. 4,325,124: SYSTEM FOR CONTROLLING THE DIRECTION OF THE MOMENTUM VECTOR OF A GEOSYNCHRONOUS SATELLITE.
in a different context, the American U.S. Pat. No. 3,945,148: SATELLITE ROTATION BY RADIATION PRESSURE, which proposes the use of the solar pressure on the solar generator wings, which are appropriately oriented to rotate the satellite in order to stabilize it.
In the following description the term "solar generator" refers to the combination of both solar wings, the term "solar generator wing" designating the systems that can be oriented by the drive motors, namely:
the photovoltaic elements of the solar generator,
the structure supporting these elements,
the mechanisms associated with this structure which:
hold it in the stowed position until the satellite reaches its orbital configuration,
deploy it and maintain it in the deployed configuration,
all the additional elements which, in the orbital configuration, are fixed to the structure and which have various roles, including (for example):
heatshields which are used to limit heat loss from the satellite during phases in which the solar generator is not fully deployed,
surfaces which improve the luminous flux impinging on the photovoltaic elements (shadow uniformization screen, for example).
In some cases a satellite has deployable heatsinks which can also be used as surfaces exposed to the solar radiation.
The main advantage of this principle is its use of existing equipment on the satellite:
the solar generator wings as the surfaces exposed to the solar radiation, and
the solar generator drive motors, as the actuators.
It has disadvantages, however:
no torque is generated about the pitch axis (Y) on which there is therefore no compensation of any disturbances,
it is not possible to generate any torque about the transverse axis P without simultaneously generating a torque about the solar axis S, which rules out its use for automated control at all times of day and prevents the balancing out of some combinations of disturbing torques,
significant torque about the P axis can only be generated at the cost of significant loss of power from the solar generator.
The French patent FR-2.530.046 PROCEDE ET DISPOSITIF DE COMMANDE D'ATTITUDE POUR SATELLITE GEOSYNCHRONE proposes an improvement to this principle by adding to the sides of the solar generator wings lateral vanes fixed symmetrically relative to the satellite-Sun axis passing through the centre of the satellite. This retains the use of the solar generator drive motors as actuators and makes it possible to obtain all combinations of torque about the S and P axes, with loss of power from the solar generator limited to acceptable values. The major disadvantage of this concept is that it systematically adds to the torque generated in the roll-yaw plane a disturbing torque about the pitch axis Y due to the offset between the centre of the lateral vane surfaces and the pitch axis Y. This Y-axis torque is subject to fluctuations which can be of large amplitude, with a direct correlation in direction and in amplitude to variations in the roll-yaw torque applied about the P axis, around a null mean value. This disturbing torque increases the consumption of propellant and/or the size of the wheels used for pitch control.
An object of the invention is to meet not only the objective of the document FR-2.530.046 with regard to attitude control in the roll-yaw plane, but further to enable as complete compensation as possible of external disturbance about the pitch axis, without significant increase in mass or reduction of reliability. It is therefore directed to reducing the amplitude of these fluctuations, while allowing a choice of the mean value of said fluctuations with a possibly non-null value adapted to compensate on average the external disturbances about the Y axis.
Another object of the invention is, for the same surface area of the additional vanes as in the patent FR-2.530.046 and for the same maximum amplitude of depointing relative to the Sun, to increase the torques that can be achieved about the P axis and so to widen the range of satellites adapted to be controlled in attitude using the solar radiation pressure, in particular to confer the advantages of the previously mentioned patent.